4 Types of Isomers 1. Structural Isomers/(Constitutional) 2. Geometric Isomers/(Cis/Trans) 3. Optical Isomers A. Enantiomers B. Diastereomers
Dec 24, 2015
4 Types of Isomers
1. Structural Isomers/(Constitutional)2. Geometric Isomers/(Cis/Trans)3. Optical Isomers
A. EnantiomersB. Diastereomers
C4H10 C4H10
Stereoisomer/(Geometric)Constitutional/(Structural)
Geometric/Cis-Trans
EnantiomersDiastereomers
Isomers
Same – FormulaDiff. – Structure Chemical Properties Physical Properties Biological Properties
Same – Formula StructureDiff. – Geometry (C=C) Chemical Properties Physical Properties Biological Properties
Same – Formula Structure Chemical Properties Physical PropertiesDiff. – Geometry Optical Rotation Biological Properties
Same – Formula StructureDiff. – Geometry Chemical Properties Physical Properties Biological Properties
Cl
Cl
Cl
Cl
4 Types of Isomers
OHH
CH3
OHH
O
OHH
CH3
HOH
O
OHH
CH3
OHH
O
HOH
CH3
HOH
O
Structural Isomers
• Chemical Formula • Structure• Chemical Properties• Physical Properties• Biological Properties
Same Different
C4H10 C4H10
Example
Geometric Isomers
• Chemical Formula• Structure
• Geometry (around C=C)• Chemical Properties• Physical Properties• Biological Properties
Same Different
C4H6Cl2
Example
Cl
Cl
C4H6Cl2
Cis-2,2-dichloro-2-buteneTrans-2,2-dichloro-2-butene
Cl
Cl
Optical Isomers(Enantiomers)
• Chemical Formula• Structure• Chemical Properties• Physical Properties
• Geometry – stereoisomers that are mirror images
• Optical Rotation• Biological Properties
Same Different
Definition: Chiral molecules that are mirror images of each other.
Optical Isomers(Diastereomers)
• Chemical Formula• Structure
Same• Geometry – stereoisomers that are
not enantiomers • Chemical Properties• Physical Properties• Optical Rotation• Biological Properties
Different
Definition:Chiral molecules that are not enantiomers of each other.Differ in the orientation around one chiral carbon.
OHH
CH3
OHH
O
OHH
CH3
HOH
O
Enantiomers vs.
Diastereomers
OHH
CH3
OHH
O
HOH
CH3
HOH
O
HOH
CH3
OHH
O
OHH
CH3
HOH
O
Enantiomers
Enantiomers
Diastereomers
Dia
ster
eom
ers
Dia
ster
eom
ers
Plane Polarized Light
Light normally oscillates/vibrates in every direction:
Plane Polarized Light oscillates/vibrates in only one direction:
Plane Polarized Light can interact with molecules:
Optical Rotation ofPlane Polarized Light
EnantiomerA
EnantiomerB
Chiral Molecules
Definition:Molecules that lack a plane of symmetry, and therefore have non-superimposable mirror images. Can rotate plane polarized light.
Requirements:Carbon atoms with 4 bonds to different groups.No additional plane of symmetry.
Important:Chiral molecules that are enantiomers have the same physical and chemical properties but different biological properties.
Achiral Molecules
• Molecules that possess chiral carbons but have an additional plane of symmetry
• They do not rotate plane polarized light.• Meso compounds are achiral.
CH3
H
Cl
Cl
H
CH3
Achiral/Meso
CH3
Cl
Cl
H
H
CH3
Chiral
No Plane of Symmetry
Fischer Projections
Method for drawing chiral molecules in 2D.• Always draw the most oxidized group at the top• The carbon backbone is drawn top-bottom• Swaps are allowed only for horizontal atoms
• Odd number of swaps = diff. molecule• Even number of swaps = same molecule
3D View showingtetrahedral shape
2D Representation of a 3D shape
OHH
CH3
OHH
O
Dextrorotatory andLevorotatory
Dextrorotatory • Rotates light clockwise• (+)
Levorotatory• Rotates light counter clockwise• (-)
Meso Compounds
Definition:Meso compounds have 2 (or more) chiral atoms, but do not rotate plane polarized light due to have an extra plane of symmetry
• Extra plane of symmetry• Mirror image = same molecule• Does not rotate plane polarized light
same molecule
Mirror
Number of Stereoisomer
Maximum number of stereoisomers 2n
n = number of chiral carbons
• 22 = 4• 23 = 8• 24 = 16
• Meso compounds decrease the number of isomers formed
Racemic Mixtures
Definition:A mixture of equal amounts of two enantiomers
• Does not rotate plane polarized light (the 2 enantiomers cancel each other out)
Same/Different
Two Fischer projections:• Even # swaps = same molecule• Odd # swaps = different molecule
OH
ClBr
CH3
OH
BrCl
CH3
OH
CH3Cl
Br
One Swap One Swap
Same Same
Different
Identifying Chiral Carbons
Definition: Chiral carbons are bonded to 4 different groups of atoms• no C=C or C≡C• watch out for same groups like methyl/ethyl
chiral 4 different groups• H• methyl• ethyl• propyl
Enantiomers
Definition:Chiral molecules that are non-superimposable mirror images of each other.Same chemical and physical propertiesDifferent optical activity and biological properties
Mirror